Hot plug devices offer flexibility for adding function boards to a main system while the system is operating. Hot plug systems refer to systems that may receive additional plug-in devices without terminating power to the system.
A problem with hot plug systems is that input capacitors on the device to be plugged in, which is commonly referred to as a function board, draw a large amount of current from a voltage source located on the main system. This current is commonly referred to as in-rush current. A large current in-rush creates source voltage deviation, which is sometimes referred to as "undershoot." Undershoot may cause malfunctioning of the main system.
A prior method for addressing this problem utilizes an in-rush current limiter in series with the in-rush current. The in-rush current limiter is implemented by a negative temperature characteristic resistor. During hot plug-ins, the initial resistance of the negative temperature characteristic resistor is large, and the in-rush current generates heat in the resistor. This generated heat reduces the resistance of the negative temperature characteristic resistor to an operating resistance that is determined by the normal operating current. This method is somewhat suitable for low current applications. However, for high current applications, the operating resistance in the negative temperature characteristic resistor is too high and causes significant power loss. Furthermore, for either type of application, negative temperature characteristic resistors are expensive, increasing the cost of systems utilizing such resistors.